Neurotrophic factors are naturally-occurring proteins that regulate neuronal survival during development and regulate plasticity and structural integrity of the adult nervous system. Neurotrophic factors can be isolated from neural tissue and from non-neural tissue. During the last twenty years, many neurotrophic factors have been discovered. These neurotrophic factors can be classified into superfamilies, families, subfamilies and individual species based on their structure and function.
Neurotrophic factor superfamilies include the fibroblast growth factor (FGF) superfamily, the neurotrophin superfamily, and the transforming growth factor-β (TGF-β) superfamily. The glial cell line-derived neurotrophic factor (GDNF)-related ligands are a family of proteins within the TGF-β superfamily. GDNF-related ligands include GDNF, persephin (PSP), neurturin (NTN) and neublastin (NBN; known as artemin or enovin). Members of the GDNF-related ligand family are distinguished by, among other things, their seven conserved cysteine residues. These residues form intramolecular and intermolecular disulfide bridges and give rise to the tertiary and quaternary structure of the dimerized polypeptide ligand. Members of the family also share the ability to induce signaling through a multicomponent receptor complex consisting of a glycosylphosphatidylinositol (GPI)-anchored co-receptor of the GFRα family, a member of the GDNF-related ligand subfamily, and the RET tyrosine kinase receptor.
Activated RET initiates a signal transduction cascade that is responsible, at least in part, for the downstream effects of GDNF-related ligands. Accordingly, activation of RET may represent one desirable aspect of a therapy which acts through a GFRα receptor pathway to affect downstream cellular processes.
Neublastin is classified within the GDNF family because it shares regions of homology with other GDNF ligands including the seven cysteine motif (e.g., as described in EP02/02691, PCT publications US02/02319 and US02/06388), and because of its ability to bind to, and activate, the RET receptor as part of a GFRα complex. Specifically, neublastin is highly selective for binding to the GFRα3-RET receptor complex. In that respect, neublastin contains unique sub regions in its amino acid sequence as compared with other members of the GDNF-related ligand family.
Current data suggest that neublastin may have a protective and regenerative role in the peripheral and central nervous systems and, as a result, may be useful as a therapeutic agent for neurodegenerative disorders. For example, data suggest that neublastin may have survival promoting effects on cultured sensory neurons from dorsal root ganglia and from trigeminal ganglia, and on cultured substantia nigra dopaminergic neurons (Baloh et al., Neuron 21: 1291-1302 (1998)). It therefore appears that neublastin may promote survival of neuronal populations including sensory and dopaminergic neurons. This is important because the degeneration and dysfunction of neurons has been associated with disease states. For example, sensory and dopaminergic neuron pathologies underlie peripheral neuropathy and Parkinson's disease, respectively.
Therefore, administration of neublastin may be useful, for example, in the treatment of diseases associated with neuronal degeneration and dysfunction. However, neublastin is rapidly cleared by the body, which may affect the neublastin dosing paradigm required in therapeutic applications. Thus, a need exists for modified neublastin polypeptides with enhanced bioavailability. Accordingly, it is an object of the present invention to identify modified forms of neublastin which exhibit enhanced bioavailability.